1. Field of the Invention
This invention relates to an apparatus for manufacturing a netted structure which receives part of a filament assembly of molten resin before it lands on a water surface of a water tank and narrows the thickness of the filament assembly so as to form surface layers of the netted structure, and a method for manufacturing a netted structure.
2. Brief Description of the Related Arts
A netted structure manufactured by helically and randomly entangling a plurality of filaments made of a thermoplastic resin such as polyethylene as a main material, and partly and thermally bonding the filaments are known in prior art. Such a netted structure, carrying countless numbers of voids among filaments, is used for a bedding mat, a cushion or a buffer due to their high shock absorbing property.
A method for manufacturing such a netted structure first stores a molten resin in a container called a die. A metal plate having a number of holes is provided on a lower face of the die. Filaments are formed by extruding the molten resin through those holes and free-fall to a water tank located below. After landing on water, the filaments are varied in velocity by buoyancy and resistance of water, are solidified by being cooled and thus are helically and randomly entangled and partly and thermally bonded in water as described above.
By the way, it is known that making the apparent density of the surface layers of the netted structure higher than that of the inner layer has a good effect for comfort and permanent set-in fatigue resistance when using the netted structure as a bedding mat or a cushion. For this reason, metal plates called chutes may be provided, in some cases, above the water tank to receive the filaments before they land on the water of the water tank and to guide them to the water tank.
The chutes, which are inclined, and on the inclined surfaces of which flowed water makes uniform water layers, await the filaments. Part of the filaments extruded from the holes are guided to the water tank with sliding on the inclined surfaces of the chutes while being entangled and thermally bonded on the surfaces of the chutes as described above and at the same time being narrowed toward an inner layer so as to form surface layers with a higher apparent density. At the same time, the outer surfaces of the surface layers are made smooth while sliding on the smooth chutes. Therefore, compared to a netted structure with not-smoothed helices of filaments on its surface, when covering the netted structure for example, bothersome hooking of the filaments to a cover can be prevented and a deterioration of strength due to break of bonding of filaments being hooked and pulled can be also prevented.
Formation of the surface layers of such netted structures relates significantly to the conditions of the water layer and friction of the inclined surfaces of the chutes. Quality of the surface layers depends on these conditions. For example, Japanese Patent No. 4181878 discloses a method of providing water-permeable sheets covering the surfaces of the chutes and supplying cooling water between the surfaces of the chutes and the water-permeable sheets. This method can form a uniform cooling water layer on the surface of the chute by suctioning water with the water-permeable sheet. The formed cooling water layer can buffer the impact of landing of the filaments, and at the same time, friction resistance of the water-permeable sheet can control slide of the filaments so as to form adequate loops (helical and random entangling of the filaments).